1. Technical Field
The present disclosure relates to a battery degradation determination device, a battery degradation determination method and a battery degradation determination system, which determine a degree of battery degradation based on AC impedance measurement data for a battery.
2. Related Art
A battery such as a secondary battery or a fuel cell is degraded depending upon a number of recharging cycles, a usage environment, a storage condition, and the like and becomes unserviceable when the service life thereof is eventually exceeded. For this reason, various methods for determining a degree of battery degradation have previously been proposed, and for example, a method has been known in which the degree of degradation is determined based on a circuit constant obtained by fitting AC impedance measurement data of the battery to an equivalent circuit model of the battery.
As an equivalent circuit model, an RC two-stage model has widely been used in which two stages of RC parallel circuit blocks are connected to a resistance R1 as shown in FIG. 9. The unknown circuit constants in the RC parallel circuit block are R and C, the number of which is two per one stage, and it is possible to easily perform fitting because the number of unknown circuit constants in the RC two-stage model are not so large.
An AC impedance characteristic is typically represented by a Nyquist diagram as a complex impedance characteristic. As shown in FIG. 10, when the RC two-stage model is used as the equivalent circuit model, fitting error tends to be large for a battery with a deformed arc-shaped complex impedance characteristic. Here, in FIG. 10, a broken line represents an actually measured complex impedance characteristic, while a solid line represents a complex impedance characteristic based on circuit constants obtained by fitting to the RC two-stage model.
An actual battery rarely has a complex impedance characteristic represented by perfect semicircular shapes and has a deformed arc-shaped complex impedance characteristic as shown by the broken line in FIG. 10 in many cases. For this reason, it cannot be said that a degradation determination result based on the circuit constants obtained by fitting to the RC two-stage model is always sufficiently appropriate.